Chapter 12 Sequences and Series - 12.4 Find Sums of Infinite Geometric Series - 12.4 Exercises - Mixed Review - Page 825: 48

$a_n=216-18n$

Here, we have $a_n= a_1+d(n-1)$ for the Arithmetic series. The common difference between the successive terms is $d=-18$ First term is: $a_8=72$ Here, we have $a_8= a_1+7d$ and $a_1=a_8-7d=72-7 \times (-18)=198$ $a_n=198+(-18) \times (n-1)=198-18n+18$ Hence, $a_n=216-18n$